1. Field of the Invention
This invention relates generally to injection molding and more particularly to an injection molding nozzle having an integral electrical heating element surrounded by layered dielectric insulation.
2. Related Art
Heaters for injection molding and hot runner applications are known in the prior art, as demonstrated amply by the following U.S. Pat.: U.S. Pat. Nos. 2,991,423, 2,522,365, 2,769,201, 2,814,070, 2,875,312, 2,987,300, 3,062,940, 3,550,267, 3,849,630, 3,911,251, 4,032,046, 4,403,405, 4,386,262, 4,557,685, 4,635,851, 4,644,140, 4,652,230, 4,771,164, 4,795,126, 4,837,925, 4,865,535, 4,945,630, and 4,981,431.
Heaters are of course also amply known in non-injection molding applications, as shown for example in U.S. Pat. Nos. 2,088,586, 2,378,530, 2,794,504, 4,438,322 and 4,621,251.
There are in general three types of heaters known for use in the hot runner nozzles. The first is so-called “integral heaters” which are embedded or cast in the nozzle body. Examples of such nozzles are disclosed in the following patents: U.S. Pat. Nos. 4,238,671, 4,386,262, 4,403,405 and EP 765728. The second is so-called “independent external heaters” which have their own support and that can be removed and replaced. Essentially, in such a design, shown in FIG. 1a, the heating element H is external to the nozzle body N. Heating element H comprises a resistance wire W surrounded by electrical insulating material E and is encased in a steel casing C. Examples of such nozzles are disclosed in the following patents: U.S. Pat. Nos. 3,553,788, 3,677,682, 3,831,004, 3,912,907, 4,588,367, 5,360,333, 5,411,393, 5,820,900, EP 748678, EP 963829 and EP 444748. The third is so-called “attached external heaters” which are positioned spirally around the exterior of the nozzle or the nozzle tip but cannot be removed therefrom by reason of being brazed or embedded in the nozzle surface. Referring to FIG. 1b, heating element H′ is embedded in a groove G′ in nozzle body N′. Examples of such nozzles are disclosed in the following patents: U.S. Pat. Nos. 4,557,685, 4,583,284, 4,652,230, 5,226,596, 5,235,737, 5,266,023, 5,282,735, 5,614,233, 5,704,113 and 5,871,786.
Electrical heaters have been also used in the design of the so-called hot runner probes. Unlike the hot runner nozzles, the hot runner probes do not comprise the melt channel. The probes are located inside the melt channel of the nozzle and thus create an annular flow. The melt is heated from the inside and this heating approach is not applicable to all materials and applications. Examples of such nozzles are disclosed in the following U.S. Pat. Nos. 3,800,027 3,970,821, 4,120,086, 4,373,132, 4,304,544, 4,376,244, 4,438,064, 4,492,556, 4,516,927, 4,641,423, 4,643,664, 4,704,516, 4,711,625, 4,740,674, 4,795,126, 4,894,197, 5,055,028, 5,225,211, 5,456,592, 5,527,177 and 5,504,304.
Injection molding nozzles having integral heaters typically have electrical heating elements, wound spirally around the nozzle, which offer an efficient response to the many critical process conditions required by modern injection molding operations. There has been a continuous effort in the prior art, however, to improve the temperature profile, the heating efficiency and durability of such nozzles and achieve an overall reduction in size. Most of these efforts have been aimed at improving the means of heating the nozzle.
For example, U.S. Pat. No. 5,051,086 to Gellert discloses a heater element brazed onto the nozzle housing and then embedded in multiple layers of plasma-sprayed stainless steel and alumina oxide. To avoid cracking of the ceramic layers caused by excessive thickness and the differing thermal properties of the ceramic and the stainless steel, Gellert employs alternating thin layers of stainless steel and alumina oxide. The heating element of Gellert is a nickel-chrome resistance wire (i.e. see W in FIGS. 1a and 1b herein) extending centrally through a refractory powder electrical insulating material (i.e. see E in FIGS. 1a and 1b), such as magnesium oxide, inside a steel casing (i.e. see C in FIGS. 1a and 1b). The heating element is integrally cast in a nickel alloy by a first brazing step in a vacuum furnace, which causes the nickel alloy to flow by capillary action into the spaces around the heater element to metallurgically bond the steel casing of the element to the nozzle body. This bonding produces very efficient and uniform heat transfer from the element to the nozzle body.
Nozzles with this type of electrical heaters, however, are often too big to be used in small pitch gating due to the size of the insulated heater required. These heaters are also generally expensive to make because of complex machining required. Also, the manufacturing methods to make these nozzle heaters are complex and therefore production is time consuming.
U.S. Pat. No. 5,955,120 to Deissler which discloses a hot runner nozzle with high thermal insulation achieved by coating the electrical heater with layers of a thermally insulation materials (mica or ceramic) and high wear resistance material (titanium). Like Gellert, the heater element of Deissler has its own electrical insulation protection and thus can be placed in direct contact with the metallic nozzle body (see FIG. 2 of Deissler). Also the heater element of Deissler is attached to the nozzle by casting (brazing) a metal such as brass. Deissler is thus similar to Gellert in that it discloses an insulated and brazed heater element. Again, as with Gellert, such a device requires many additional steps to braze and insulate the heater and is therefore time consuming. Also, as with Gellert, the use of an insulated element makes the size of the heated nozzle not well suited for small pitch applications.
In an attempt to reduce nozzle size, U.S. Pat. No. 5,973,296 to Juliano shows a thick film heater applied to the outside surface of an injection nozzle. The nozzle heater comprises a dielectric film layer and a resistive thick film layer applied directly to the exterior cylindrical surface of the nozzle by means of precision thick film printing. The thick film is applied directly to the nozzle body, which increases the nozzle's diameter by only a minimal amount. Flexibility of heat distribution is also obtained through the ability to apply the heater in various patterns and is, thus, less limited than spiral designs.
There are limitations to the thick film heater, however. Thermal expansion of the steel nozzle body during heating can cause unwanted cracking in the film layers due to the lower thermal expansion of the film material. This effect is particularly acute after a large number of injection cycles. The cracks could affect the resistive film heater because it is not a continuous and homogeneous material (as is a wire), but rather the fine dried powder of the conductive ink, as disclosed in Juliano '296.
Another heated nozzle design is disclosed in U.S. Pat. No. 4,120,086 to Crandell. In one embodiment, Crandell '086 discloses an electrically heated nozzle having an integral heater comprising a resistance wire heater disposed between two ceramic insulating layers. The Crandell '086 nozzle is made by wrapping a metal nozzle body with flexible strips of green (ie. unsintered) ceramic particles impregnated in heat dissipatable material, subsequently winding a resistance wire heating element around the wrapped green layer, wrapping a second layer of the flexible strips of green ceramic particles thereover, heat treating the assembly to bake out the heat dissipatable material and sinter the ceramic particles together, and then compacting the assembly to eliminate air voids in the assembly. In U.S. Pat. No. 4,304,544, also to Crandell, the inventor further describes the flexible green ceramic strips as comprising a body of green ceramic insulator particles which are impregnated in a heat dissipatable binder material. In the green state, such strips are pliable and bendable, permitting them to be wrapped around the metal nozzle core, but when baked, the strips become hard and the particles agglomerate into a mass.
The Crandell '086 and '544 nozzle has relatively thick ceramic layers, employs an awkward process for applying the ceramic layers and requires additional heat treatment steps in fabrication. Crandell '086 concedes that the baking step is time consuming (see column 5, lines 20-25) and therefore admits that the design is less preferable than other embodiments disclosed in the patent which do not utilize this method. Also, as mentioned above, it is desirable to reduce nozzle size, which is not possible with the thick ceramic strips of Crandell '086 and '544.
The use of ceramic heaters for both hot runner nozzle heaters and hot runner probe heaters is also disclosed in U.S. Pat. No. 5,504,304 to Noguchi. Noguchi, like Juliano, uses a printing method to form an electrical resistive wire pattern of a various pitch from a metal or a composite paste. A ceramic heater embodiment for a nozzle probe (shown in FIG. 1 of Noguchi) is made by printing various electrical resistive patterns shown in FIGS. 3-4 of Noguchi. Noguchi discloses a method whereby a mixture of insulating ceramic powder such as silicon carbide (SiC), molybdenum silicide (MoSi2) or alumina (Al2O3) and silicon nitride (SiN), and electrically conductive ceramic powder such as titanium nitride (TiN) and titanium carbide (TiC) is sintered and kneaded into a paste, which is then printed in a snaking manner on the external surface of a cylindrical insulating ceramic body, as shown in FIG. 3 of Noguchi. The printing state is made denser in certain areas and, by so controlling the magnitude of the so-called “wire density,” a temperature gradient is given to the heater. The heater pattern can be formed using metals such as tungsten, molybdenum, gold and platinum. A ceramic heater embodiment for a hot runner nozzle is also disclosed in Noguchi (see FIG. 9 of Noguchi). This self-sustained ceramic heater is also made by wire-printing using the same paste or metals. The heater is placed over the nozzle body and is then sintered and kneaded into a paste comprising a mixture of insulation ceramic powder such as silicon carbide, molybdenum silicide or alumina and conductive ceramic powder such as titanium nitride and titanium carbide. The paste is printed in a single snaking line on the part where, again, the heater pattern is formed by applying temperature gradients by varying the magnitude of wire density across the part.
Although Noguchi introduces a wire-printing method to achieve a certain heat profile along the nozzle it does not teach or show how this wire-printing method is actually implemented. More detailed information about this wire-printing method is provided by the patentee's (Seiki Spear System America. Inc.) catalogue entitled “SH-1 Hot Runner Probe” (undated). According to the catalogue, the circuit pattern, which provides the resistance for heating, is screen printed direction onto a “green” or uncured ceramic substrate. The flexible “green” substrate with the printed circuit is wrapped around an existing ceramic tube and the complete unit is fired and cured to produce a tubular heater. The resistive circuit pattern is encased within the ceramic between the tube and the substrate and has no exposure to the outside atmosphere. The thermocouple is inserted through the centre of the tubular heater and positioned in the tip area. Thermocouple placement in the probe tip gives direct heat control at the gate. The ceramic heater unit is then fixed outside the probe body. Thus, this Seiki Spear method of making a ceramic heater body according to Noguchi including a printed-wire is similar to the method disclosed in Crandell '086, with the exception that Crandell uses a self-sustained resistance wire wound spirally around the nozzle between two “green” ceramic layers. As with Crandell, as well, an additional sintering step is required to sinter the green ceramic layers.
Accordingly, there is a need for a heated nozzle which overcomes these and other difficulties associated with the prior art. Specifically, there is a need for a heated nozzle which is simpler to produce and yields a more compact design.